1. Field of the Invention
The present invention relates to a thin-film magnetic head having microwave magnetic exciting function for recording data signal onto a magnetic recording medium that has a large coercivity for thermally stabilizing the magnetization, and to a magnetic recording and reproducing apparatus with this thin-film magnetic head.
2. Description of the Related Art
With the demand for higher recording density of a magnetic recording and reproducing apparatus such as a magnetic disk drive apparatus, each bit cell in a magnetic recording medium for recording digital information has been miniaturized and, as a result, a signal detected by a read head element in the thin-film magnetic head sways due to such as thermal fluctuation. This causes deterioration in a signal-to-noise ratio (S/N), and in the worst case, the signal detected by the read head element may disappear.
It is effective for a magnetic recording medium adopted for the perpendicular magnetic recording scheme that is recently put to practical use to increase perpendicular magnetic anisotropy energy Ku of a magnetic recording layer in this recording medium. On the other hand, a thermal stabilization factor S that corresponds to the thermal fluctuation is represented by the following equation (1) and is necessary to have in general 50 or more:S=Ku·V/KB·T  (1)where Ku is perpendicular magnetic anisotropy energy, V is a volume of crystal grains that form the recording layer, kB is the Boltzmann constant, and T is an absolute temperature.
According to the so-called Stoner-Wohlfarth model, an anisotropy magnetic field Hk and a coercivity Hc of the recording layer are represented as the following equation (2):Hk=Hc=2Ku/Ms  (2)where Ms is a saturated magnetization of the recording layer.
The coercivity Hc increases with the increase in the perpendicular magnetic anisotropy energy Ku. In a normal recording layer, however, Hk is higher than Hc.
In order to perform desired inversion of magnetization in the magnetic recording layer in response to data sequence to be written, a write head element of the thin-film magnetic head is required to apply a recording magnetic field having a precipitous rising edge and a level up to about the anisotropy magnetic field Hk of the recording layer. In a hard disk drive (HDD) apparatus adopting the perpendicular magnetic recording scheme, a write head element with a single pole is used so that a recording magnetic field is applied perpendicular to the recording layer from an air-bearing surface (ABS) of the element. Since an intensity of this perpendicular recording magnetic field is proportional to a saturated magnetic flux density Bs of the soft magnetic material that forms the single pole, a material with a saturated magnetic flux density Bs as high as possible is developed and is put into practical use for the single pole. However, the saturated magnetic flux density Bs has the practical upper limit of Bs=2.4 T (tesla) from a so-called Slater-Pauling curve, and a recent value of the saturated magnetic flux density Bs of soft magnetic material closes to this practical upper limit. Also, in order to increase the recording density, the thickness and width of the single pole have to decrease from the present thickness and width of about 100-200 nm causing the perpendicular magnetic field produced from the single pole to more lower.
As aforementioned, due to the limit of recording ability of the write head element, high-density recording becomes difficult now. To overcome such problems, suggested is so-called thermal assisted magnetic recording (TAMR) scheme for recording a magnetic signal on a recording layer of the magnetic recording medium under conditions where the recording layer is irradiated by a laser beam for example to increase the temperature and to lower the coercivity Hc of the magnetic recording layer.
Japanese patent publication No. 2001-250201 discloses a TAMR technique in which electrons are radiated to a magnetic recording medium from an electron radiation source to heat a recording part in the magnetic recording medium so that the coercivity Hc is lowered and thus it is possible to record magnetic information on the medium using a magnetic write head.
U.S. Pat. No. 7,133,230 B2 discloses another TAMR technique in which a laser beam from a semiconductor laser element formed in a perpendicular magnetic recording head is irradiated to a scattering member or near-field light probe formed in contact with a main pole of the head so as to produce a near-field light, and the produced near-field light is applied to the magnetic recording medium to heat it and rise the temperature.
However, there are various difficulties and problems in these TAMR techniques. For example, (1) a structure of the thin-film magnetic head becomes extremely complicated and its manufacturing cost becomes expensive because the head has to have both a magnetic element and an optical element, (2) it is required to develop a magnetic recording layer with a coercivity Hc of high temperature-dependency, (3) adjacent track erase or unstable recording state may occur due to thermal demagnetization during the recording process.
Recently, in order to increase sensitivity of a giant magnetoresistive effect (GMR) read head element or a tunnel magnetoresistive effect (TMR) read head element, study of spin transfer in electron conductivity is made active.
US patent publication Nos. 2007/0253106 A1 and 2008/151436 A1, and J. Zhu, “Recording Well Below Medium Coercivity Assisted by Localized Microwave Utilizing Spin Transfer”, Digest of MMM, 2005 disclose application of this spin transfer technique to the inversion of magnetization in a recording layer of a magnetic recording medium so as to reduce a perpendicular magnetic field necessary for the magnetization inversion.
According to this scheme, an alternating magnetic field of high frequency is applied to the magnetic recording medium in a direction parallel to its surface together with the perpendicular recording magnetic field. The frequency of the alternating in-plane magnetic field applied to the magnetic recording medium is an extremely high frequency in the microwave frequency band such as several GHz to 40 GHz, which corresponds to a ferromagnetic resonance frequency of the recording layer. It is reported that, as a result of simultaneous application of the alternating in-plane magnetic field and the perpendicular recording magnetic field to the magnetic recording medium, a perpendicular magnetic field necessary for the magnetization inversion can be reduced to about 60% of the anisotropy magnetic field Hk of the recording layer. If this scheme is put in practical use, it is possible to increase the anisotropy magnetic field Hk of the recording layer and thus it is expected to greatly improve the magnetic recording density without utilizing the complicated TAMR system.
However, according to the conventional technique, since the microwave magnetic field radiation means consists of a write coil wound around a magnetic body or of an individual sub-coil separately formed from the write coil, if a frequency of a microwave signal to be applied more increases, radiation of the microwave magnetic field will occur at a part of the write coil or the sub-coil itself and thus it is impossible to radiate microwave magnetic field toward the magnetic recording medium even when the power supply is increased. Therefore, if the frequency of the microwave signal to be applied more increases, it is quite difficult to increase the anisotropy magnetic field Hk of the recording layer.
The applicant has already proposed a method using a plane-structure waveguide such as a coplanar waveguide (CPW) or an inverted micro strip waveguide (I-MLIN) as for a microwave magnetic field emission means in the microwave-assisted magnetic recording system (for example, Japanese Patent Application No. 2008-242400, filed on Sep. 22, 2008).
According to this method, because a distance between a ground conductor or a grand electrode and a line conductor or a signal electrode minimizes, a strong high-frequency electric field or a strong high-frequency magnetic field can be obtained. This operation uses Coulomb's law in charge distribution, in which the electric field E increases inversely proportional to square of distance r between the electrodes, that is, E=kQ/r2, wherein E is an electric field, Q is an electric charge, k is a constant of proportion, and r is a distance between the electrodes. In other words, using is that a strong electric field and magnetic field can be provided in proportion to the square of a distance when the distance minimizes.
FIGS. 14a and 14b illustrate configurations of CPW and I-MLIN as a microwave magnetic field emission means in the microwave-assisted magnetic recording system, respectively. Both of FIGS. 14a and 14b indicate in sections that are perpendicular to the track-width direction of a thin-film magnetic head. Therefore, right and left directions in the figures correspond to the relative moving direction of a magnetic recording medium, and a perpendicular direction to a plane of the figures corresponds to the track-width direction.
In FIG. 14a, reference numeral 140 denotes a line conductor of CPW, 141 and 142 denote a main pole layer and an auxiliary pole layer of a magnetic write head element arranged at top and bottom side (both sides in FIG. 14a) in a laminated direction of the line conductor 140, and 143 denotes a magnetic recording medium, respectively. In this configuration, the magnetic recording medium 143 is not grounded, and the main pole layer 141 and the auxiliary pole layer 142 are grounded to form a ground conductor of CPW. According to such configuration of CPW, when a microwave current is supplied, electrical flux lines 144 are applied to the ground conductor 141 that is arranged at top and bottom sides in the laminated direction of the line conductor 140 but no electrical flux line is directly applied to the magnetic recording medium. Thus, magnetic field 145 produced to flow perpendicular to the electrical flux lines 144 also reduces itself. In other words, the resonance magnetic field 145 in a longitudinal direction (track direction in the magnetic recording medium surface or substantially in the magnetic recording medium surface) that is perpendicular to the direction of the electrical flux lines 144 toward the surface of the magnetic recording medium 143 becomes weak. This resonance magnetic field 145 is a high-frequency magnetic field in a microwave band at or around a ferromagnetic resonance frequency FR of a magnetic recording layer of the magnetic recording medium 143. However, since the resonance magnetic field 145 is directed to the longitudinal direction or the track direction, even if this magnetic field 145 is applied to the magnetic recording layer during the write operation, it is impossible to reduce a coercive force of the magnetic recording layer so as to decrease the write magnetic field intensity, which is necessary for writing, in a direction that is perpendicular to or substantially perpendicular to the magnetic recording medium surface.
Also, in FIG. 14b, reference numeral 150 denotes a line conductor of I-MLIN, 151 and 152 denote a main pole layer and an auxiliary pole layer of a magnetic write head element arranged at top and bottom side (both sides in FIG. 14b) in a laminated direction of the line conductor 150, and 153 denotes a magnetic recording medium, respectively.
In this case, the magnetic recording medium 153 is grounded, and is utilized as a ground conductor of I-MLIN. According to such configuration of I-MLIN, since it is designed that the ground conductor or the magnetic recording medium 153 located below the line conductor 150 becomes the return of the electrical flux lines when a microwave current is supplied, most of the electrical flux lines 154 are directly applied to the magnetic recording medium 153, and also the magnetic field 155 is concentrated toward a lower position at which the magnetic recording medium 153 exists without laterally spreading. It should be noted however that this operation will occur only when a distance between the line conductor 150 and each of the main pole layer 151 and the auxiliary pole layer 152 arranged at top and bottom sides in the laminated direction of the line conductor 150 is larger than a distance between the line conductor 150 and the magnetic recording medium 153 that is the grand conductor.
To maximize the power emitted from the line conductor 150 in the I-MLIN, it is necessary to collect the electrical flux lines from the line conductor 150 and to emit them to the magnetic recording medium 153. However, the shape and the size of the line conductor 150, which can concentrate the electrical flux lines from the line conductor 150 to the magnetic recording medium 153, have never been brought under review.